Flexography is a type of relief printing that uses flexible sheets of photopolymer to transfer an image onto a substrate. The photopolymer sheet is generally exposed to suitable radiation (usually actinic or visible or UV light) through a negative or mask that blocks selected portions of the photopolymer from the light. The portions of the photopolymer that are exposed are crosslinked or hardened and the non-exposed portions are removed because of their solubility in various organic solvents in a “washout” or development process. The resulting developed flexographic printing plate has a raised relief image in those portions that were exposed to radiation. The flexible printing plate is usually wrapped around a cylinder on a printing press and used to transfer ink to a suitable substrate such as papers, films, fabrics, ceramics, and other materials.
While the non-exposed photopolymer may be soluble in a variety of organic solutions, only some of those solutions are the best developers that do not damage or swell the crosslinked portions while cleanly removing the non-crosslinked portions. Swelling will eventually cause the relief image to deteriorate in the processing bath and the processing solution will have to be changed frequently due to the build-up of sludge.
The development or washout of imaged photopolymerizable flexographic printing plate precursors is well known, for example as described in U.S. Pat. No. 5,354,645 (Schober et al.) and U.S. Pat. No. 6,162,593 (Wyatt et al) and U.S. Patent Application Publication 2005/0227182 (Ali et al.), and the references cited in therein. Representative processing solutions (developers) are also described in U.S. Pat. No. 6,248,502 (Eklund). Sometimes, the photopolymer compositions used in the flexographic printing plate precursors are carefully matched to the desired processing solutions.
Many known processing solutions include chlorohydrocarbons, saturated cyclic or acyclic hydrocarbons, aromatic hydrocarbons, lower aliphatic ketones, and terpene hydrocarbons. While these processing solutions are effective for the intended purpose, they have a number of disadvantages. They may act too slowly, causing swelling in the printing plates and thus damage the fine detail in the relief images, require long drying times, and have low flashpoints resulting in dangerous situations. Many of these solvents are also considered hazardous air pollutants (HAPS) and are subject to stringent governmental reporting requirements. They may also be too toxic for direct disposal into the environment or create worker safety problems with strong odors or handling problems.
More recently, there has been an emphasis on reclaiming processing solutions. A major drawback of reclamation processes is the lack of an inexpensive method for reclaiming the solvent for subsequent use. Reclamation and recycling of processing solvents generally require distillation that is energy and labor intensive. Improved reclamation processes are described for example in U.S. Pat. No. 7,326,353 (Hendrickson et al.) in which centrifugation is used alone or in combination with filtration to remove solids from reusable processing solvents.
Copending and commonly assigned U.S. Ser. No. 12/391,344 filed Feb. 24, 2009 by Bradford and Ali describes the use of microfiltration techniques for reclamation of processing solvents.
Copending and commonly assigned U.S. Ser. No. 12/545,268 filed Aug. 21, 2009 by Bradford and based on Provisional Application 61/097,358 (filed Sep. 19, 2008 by Bradford) describes flexographic printing plate washout or processing solutions containing dipropylene glycol dimethyl ether (DME) alone or in combination with various co-solvents such as alcohols and aliphatic dibasic acid ethers.
U.S. Pat. No. 6,162,593 (noted above) describes the use of diisopropylbenzene (DIPB) alone or with alcohol co-solvents in processing solutions.
Aliphatic dibasic acid is used in cleaning compositions as described in U.S. Pat. No. 4,867,800 (Disharat et al.), U.S. Pat. No. 5,011,620 (Dishart et al.), and U.S. Pat. No. 5,378,386 (Laven et al.).
Whether the processing solutions are reclaimed or not, there is a need for more environmentally friendly developing solutions that offer improvements in handling, disposal, low odor, and low levels of volatility (lower level of “volatile organic chemicals”, or “VOC” content) while still providing the effective cleanout (or washout) of non-polymerized material in an imaged flexographic printing plate precursor. It is also desirable that the processing solution be useful for a variety of photopolymer compositions so that the processing solution is versatile for multiple uses.